Allow negative values

First, this is such a fun little app! Thank you so much for sharing.

Second, there are some instances when negative weight is useful, such as when measuring the partial removal of a substance. I imagine it would be fairly simple to allow negative values when the weight is less than the tared amount.

Third, I've found that a plugged-in external hard drive works as a great capacitative base, and it's much more weight-stable than my finger!

Start by locating where the app rejects weights below the tared amount and trace how the displayed measurement is calculated. Allow negative values for partial removal while preserving the existing tare behavior, then verify that measurements below the tared amount are accepted and shown correctly.

Written by the indexing model from the issue text.